The importance of secondary batteries such as lithium secondary batteries (typically, lithium ion batteries) and nickel hydride batteries as power supplies for vehicles or power supplies for personal computers and portable terminals has grown in recent years. In particular, lithium secondary batteries that make it possible to obtain a high energy density with a light weight are expected to be advantageously used as high-output power supplies for vehicles.
The increased battery capacity is one of the characteristics that are required for secondary batteries to be used as high-output power supplies for vehicles. The use of substances that can realize a capacitance higher than that of the conventional devices as electrode active materials has been investigated as a means for fulfilling such a requirement. For example, metal compound (typically, metal oxide) materials that use Si, Ge, Sn, Pb, Al, Ga, In, As, Sb, Bi, or the like as the constituent metal elements (including semi-metallic elements; same hereinbelow) can be used in lithium secondary batteries as electrode active materials (more specifically, negative electrode active materials) that reversibly absorb and desorb lithium ions, and such materials are known to have a capacitance higher than that of the graphite materials that have been conventionally used as negative electrode active materials. Therefore, it can be expected that by using such metal compounds (typically, metal oxides) as electrode active materials, it would be possible to realize an increased capacitance of lithium secondary batteries.
However, metal compound materials (for example, metal oxide materials such as silicon oxide (SiOx)) using the aforementioned elements as the constituent elements typically have a low electric conductivity. Therefore, when such metal oxides are used as electrode active materials, it is necessary to form a conductive coating film, more specifically a coating film made of conductive carbon, on the surface of electrode active material particles made of the metal oxide, thereby ensuring conductive routes (paths) through which lithium ions or electrons can move between the electrode active material particles and between the electrode active material particles and the electrolyte solution or electrode collector.
Examples of the conventional techniques relating to such electrode active materials are disclosed in the following Patent Literature 1 to 3. Patent Literature 1 describes an electrode active material in which the surface of particles made of Si, SiO and SiO2, and a carbonaceous material is coated with carbon. Patent Literature 2 describes various particulate metal compounds coated with a carbonaceous material as compounds that can be used as a negative electrode active material for a nonaqueous lithium secondary battery. Further, Patent Literature 3 describes a negative electrode active material for a lithium secondary battery that is formed of metal complex oxide particles including an amorphous structure and a method for manufacturing such a negative electrode active material.